1. Field of the Invention
The present invention relates to image processing. More particularly, the present invention relates to a method and apparatus for interlaced image enhancement.
2. Background
Because video resolution images are decimated versions of high resolution images, the video images can contain more detail than can be displayed using conventional TV display, reproduction signals for which typically originate with NTSC or PAL format video cameras. Typical high-resolution images can be expected to contain a much greater amount of vertical high spatial frequencies than conventional TV images. This additional vertical high spatial frequency content can cause a raster scan display artifact known asxe2x80x9cinterlace flickerxe2x80x9d, where the vertical edge details (from sharp horizontal lines, for example) will flicker visibly at a 30 Hz rate on normal interlaced NTSC displays. This interlace flicker occurs because the TV monitor displays every other line of the image during the first {fraction (1/60)} second field time, and then displays the lines in between during the next {fraction (1/60)} second field time, as shown in FIG. 1. In regions of an image having significant vertical detail, the lines of the first field will be quite different from the lines of the second field. This results in the observation ofxe2x80x9cmovementxe2x80x9d in the line or edge at the 30 Hz field rate. This perceived movement is called flicker. The amount of flicker in an image depends upon the image content and the way in which the image was created.
Interlace flicker can be reduced by lowpass filtering the image in the vertical direction, namely in a direction effectively transverse to the raster scan direction, thus reducing the vertical sharpness of the image. Generally, the higher the order of the filter the better the overall quality of the resultant image. Flicker filters of order 3 to 6 are typically used to mitigate the effects of flicker. Unfortunately, the filters also distort small font lettering such that the fonts become unreadable or nearly so. Moreover, higher order filters typically require significantly more hardware and memory bandwidth.
Turning again to FIG. 1, a block diagram that illustrates an interlaced display is presented. Odd lines 10, 12, 14 and 16 are written first, followed by even lines 18, 20 and 22. Without filtering, pixel 24 is output only when even line 20 is output, thus creating the appearance of flicker. Filtering the pixel values spreads the pixel across even and odd lines as represented by pixels 26-36. While this method reduces the appearance of flicker, it also creates a blurred image.
The effect of filtering on the readability of small font sizes is illustrated in FIGS. 2A to 3B. FIG. 2A is a block diagram that illustrates an unfiltered image of the character xe2x80x9cgxe2x80x9d. For the purposes of illustration, the lines 40 comprising the character image are one pixel wide. FIG. 2B is a block diagram that illustrates the contrast levels present in the FIG. 2A image. Only two contrast levels are present in the image. One contrast level is used for the background and another contrast level is used for the lines comprising the character image. The small number of contrast levels and the difference between contrast levels provides a relatively clear image.
FIG. 3A is a block diagram that illustrates a filtered image of the character in FIG. 2A As a result of filtering the character image, the image 42 now appears to be more than one character wide, having luma or chroma values between that of the background and that of the original character image. FIG. 3B is a block diagram that illustrates the contrast levels present in the filtered image of a character in FIG. 3A. As shown by FIG. 3B, there are many contrast levels in the filtered image 42. This smoothing of the character image results in a character that is more fuzzy and consequently less readable.
Further improvements are made possible by analyzing the high frequency content of an image and storing with the image a xe2x80x9cflicker codexe2x80x9d representative of the extent, if any, to which the vertical dimension of the image is to be subjected to a low pass filtering operation in the course of preparing the image for display. However, this method is not performed real-time. Furthermore, the flicker code typically applies to a whole image and thus fails to recognize that an image may be comprised of some regions that would benefit from filtering and other regions that would not benefit.
There is a trend in the computer field for brighter displays and increased resolution. For example, increasing numbers of computer applications such as e-mail and many Windows(trademark)-based applications use relatively small font sizes such as 10 point. Also, many Web pages have brightly colored bars and boxes to attract attention. Additionally, the popularity of High definition and large screen projection TVs continues to increase. Unfortunately, the perceived effects of flicker increase with increasing display brightness and with increasing display resolution.
Accordingly, a need exists in the prior art for a method and apparatus for image enhancement for interlaced displays that mitigates the effect of flicker in real-time and provides enhanced readability of relatively small fonts.
A method for image enhancement for an interlaced display includes receiving a first group of pixels aligned about an axis, detecting a second group of pixels within the first group of pixels, each pixel of the second group of pixels having a luminous disparity between adjacent pixels less than a first threshold, determining whether each pixel within the second group of pixels is part of a line or edge, the determination including a comparison of luma or chroma disparities between neighboring pixels and filtering each pixel determined to be part of a line or edge. An apparatus for image enhancement for an interlaced display includes an interface for receiving video data, a vertical filter coupled to the video data interface to detect inter-line correlation between pixels, a horizontal filter coupled to the output of the vertical filter to detect line correlation between pixels when inter-line correlation is detected and a flicker filter coupled to the output of the horizontal filter to filter pixels having horizontal correlation.